Features

Do we have to die?

Death and taxes are said to be two of life's certainties. While the demise of the tax office is nowhere in sight, scientific advances suggest we might be able to extend our lifespans - perhaps indefinitely. But is that necessarily a good thing?

It's a difficult concept for humans to grasp. Inevitably, sooner or later, we're going to die. But do we have to get old and frail? Why can't we keep going, forever?

We're already living longer than previous generations. Medical advances, improved education, better nutrition, and increased prosperity have all combined to help us combat the ravages of time. For instance, the number of people living to 100 in Australia has soared over the last three decades; there are now 3000 centenarians, an almost 12-fold increase on the number existing in the early 1970s, according to the Australian Bureau of Statistics. The consensus of international research is that around one in every three children born in the developed world today can expect to live to 100.

And it's not a privilege only future generations will experience, says Professor James W. Vaupel, of the Max Planck Institute for Demographic Research in Germany: 'Very long lives are the probable destiny of most people alive today. For everyone in his or her 30s and younger, especially children, lifespans of 95 or 100 years will be common,' he wrote in a paper published in the journal of the US Center for Strategic and International Studies, The Washington Quarterly.

Want to live a long life? Choose your parents carefully. While environment and lifestyle certainly play a role, longevity definitely runs in families. If you have a grandparent who lived to 100, your own chances of doing the same are much higher than average. Male siblings of centenarians are 18 times as likely as their male peers to live to 100 and for female siblings of centenarians, the odds are 8.5 times higher than for other women born around the same time.

So where does the trend stop? Can we prevent ageing altogether and live indefinitely? That's a challenge because it means overcoming the harm constantly inflicted on the the multitude of tiny cells within us. Over their lifetimes, cells are subjected to a host of biological insults and injuries: infection, trauma, extremes of temperature, exposure to toxins in the environment, and damage from metabolic processes. When the damage gets to a certain point, cells self-destruct in a process known as apoptosis or cell death. Over a 12 to 24-hour period, the cell's energy powerhouses, the mitochondria, shrink. Its genetic material fragments into pieces, and eventually the cell corpse is eaten by scavengers called macrophages.

In some body tissues, these lost cells are replaced. But in other tissues, like muscle, they aren't and the muscle shrinks. In the brain, the dead cells are replaced by fibrous material. Even in tissues where cells are replaced, the replacement doesn't go on indefinitely. Cells stop dividing after a certain number of reproductions  about 50  due to specialised stretches of DNA called telomeres, found at the end of every chromosome. On each division of the cell, the telomeres shorten, until a point when apoptosis is triggered. So the telomere acts as a biological clock, limiting the supply of new cells. This means that even if an organ survives a battery of biological insults, it will eventually fail. The maximum life span for the body's tissues is thought to be around 120 years.

It's not just changes within cells we have to worry about. Other materials are important to the functioning of healthy body parts. These include proteins laid down between cells early in life. Most of these proteins have a structural role  they give a tissue elasticity (as in the artery wall) or transparency (as in the lens of the eye) or high tensile strength (as in ligaments). But as time passes, chemical cross-links form between them, reducing their strength or elasticity and interfering with their functions. And the damaged material is recycled only very slowly, if at all.

Doomed to die

It didn't have to be this way. Humans might have come up with mechanisms to ensure a longer lifespan. But there's been no evolutionary pressure to do so. Living longer doesn't necessarily increase your odds of passing on your genes  and that's what evolution's all about. From an evolutionary point of view, humans are in essence merely 'carriers' for the genetic material inside us. Once this information has been passed on to offspring, the individual is of no further use and can be allowed to die.

In fact, there's a law of diminishing returns in keeping someone alive. At some point, the energy costs to our bodies of preventing ageing, repairing cell damage, and allowing indefinite cell divisions, might be too great if we've already reproduced sufficiently to have our genes passed on. Therefore natural selection favours organisms that reproduce early and frequently, and then die. Early reproduction provides DNA with what it really needs  the ability to replicate.

It's hardly reassuring to know that we have to die because our DNA frankly doesn't give a damn! But humans  once the hapless servants of replicating chemicals  have evolved a tool that is now able to decipher and manipulate its own DNA: a sophisticated brain. For the first time in humankind's evolution, individuals have the chance to assert themselves over their DNA and control their life spans.

Masters of the Universe

Currently about 150 clinical research trials worldwide are investigating gene therapies for common diseases like cancer, heart disease, hypertension and diabetes. Some scientists think these techniques will eventually lead to treatments that can slow or stop ageing.

These might include:

Developing vaccines that stimulate the destruction of cellular waste materials by immune cells (or alternatively, inserting genes coding for enzymes that break down the waste);

Inserting genes for enzymes that break the cross-links between proteins such as collagen and elastin, that exist in the spaces around cells;

Using growth factors to stimulate cell division to rejuvenate tissues;

Using gene therapies to prevent the shortening of telomeres, and hence prolong the number of times a cell can replicate;

Replacing aged and damaged organs with new tissues grown from unspecialised cells known as stem cells;

The research trials provide tantalising glimpses of things to come. In 1998, scientists at the University of Texas extended the life of retinal cells in the laboratory by 40 per cent after inserting into retinal cells the enzyme telomerase, which lengthens telomeres.

And at Monash University in Melbourne, researchers have grown new nerve and heart tissue by inserting cells from mature tissue into primitive unspecialised cells known as stem cells  raising the prospect that aged tissue might ultimately be regenerated and transplanted into the same person, over and over again.

A brave new world?

For now, these therapies belong to a world that is some years off. But increasingly scientists are contemplating a time when they will become commonplace. And ethicists, lawyers and philosophers have joined them in pondering just what life might be like if we could live for thousands of years, or perhaps forever.

Who would get access to the technology? Would it create a divide between those who could afford these treatmenst, and those who can't and are therefore doomed to die? Would the old in positions of political power give way to the young? How would an ageing population keep innovating? Would it alter religious beliefs? And what of the effect on the planet? Would rapidly falling death rates limit the number of new births? Or would we have to introduce radical population control measures like those suggested by British ethicist, John Harris, in his book Clones, Genes and Immortality?

'There are numerous reasons why we shouldn't contemplate one everlasting generation  such as the desire to procreate, the pleasures of having and rearing children, the advantages of fresh people and fresh ideas, and the possibility of continued evolution or at least development... We might be facing a future in which the most ethical course is a sort of generational cleansing. This would involve deciding collectively how long it is reasonable for people to live in each generation, and trying to ensure that as many as possible live healthy lives of that length. We would then have to ensure that, having lived a fair innings, they died  either by suicide or euthanasia, or by programming cells to switch the aging process on again after a certain time  to make way for future generations.'

Having 'one everlasting generation' poses other problems too. All species rely on the emergence of new traits from genetic mutations to survive major environmental changes. But if we're not reproducing  or doing so only in very limited numbers  would there be enough new traits emerging to allow humans to survive? What happens if there is sudden change in our environment  a meteor, another ice age, or the emergence of a competitor for the planet's resources  to which the species must adapt? It's possible Homo sapiens, like 99.9 per cent of species in the history of the planet, would be doomed to extinction. If so, we could be the species whose brains evolved too far for their own good.